Breast cancer is a serious health problem for women worldwide. Worldwide breast cancer is the second most common form of cancer with about 1.15 million new cases diagnosed in the year 2002. A relatively favorable prognosis, compared to other cancer forms, resulted in about 410 000 deaths the same year, according to the cancer statistics published by D. M. Parkin, F. Bray, J. Ferlay and P. Pisani, entitled “Global cancer statistics, 2002” in CA: A Cancer Journal for Clinicians, vol. 55, pp. 74-108, March 2005. This figure motivates researchers all over the world in various fields of tumor detection, diagnostics and treatment. In the process of diagnosing malignant breast cancer imaging has come to play an important role. Especially X-ray mammography is used extensively even though it has some problems, mainly due to the contrast between the tumor and the surrounding tissue which could often be as low as a few percent. It is therefore of great interest to develop alternative or complimentary imaging techniques. These methods should preferably be relatively inexpensive, easy to use and they should produce images in real time.
The breast cancer treatment today is conducted on the basis that an early diagnosis, i.e. when the tumor is small, is crucial for successful treatment and is important for the long-term survival rate. Michaelson et al, “Predicting the Survival of Patients with Breast Carcinoma using Tumor Size,” published in Cancer, vol. 95, pp. 713-723, August 2002, have performed a survival analysis and developed a formula for estimating the probability of survival based on the tumor size. Their investigations show that the chance of survival for breast cancer patients is directly related to the size of the tumor at the time of diagnosis.
In the process of diagnosing malignant breast cancer imaging has come to play an important role. Especially X-ray mammography is used extensively since it fulfils most of the requirements on a good imaging method. These requirements are that it should have a high specificity and sensitivity to malignant tumors, it should not take up large amounts of manpower or time and it should be non-invasive, harmless and cost efficient.
In many screening programs X-ray mammography has proved to reduce the breast cancer mortality and today it is the routine examination method for breast cancer imaging. It is well recognized and acknowledged for its ability to detect and characterize breast tumors. Unfortunately there are some important limitations, such as high false-positive detection rate. There are reports indicating a false-positive detection rate of 2.6-15.9%, i.e. that the mammogram shows a structure that are improperly interpreted as a tumor. The same publications also conclude that the variation is strongly dependent on the experience of the radiologist. There are also estimates that malignant tumors are found in 10-50% of the patients called back for breast biopsy based on the findings in the mammogram.
The false-negative rate is also significant and reported to be 4-34%, depending on the definition of a false-negative mammogram. Also here is has proven that the experience of the radiologist and the population selected for the study strongly influences the estimated percentage rate. In general it is considered that about 5-15% of the malignant breast cancers are not detected. An important contribution to these numbers comes from the difficulty in imaging radiographically dense breasts, containing a large amount of fibroglandular tissue. About 25% of all women, especially young women, have this type of breasts and other imaging methods than X-ray mammography plays a more important role here. One of the reasons for failure to detect some tumors with the X-ray mammographic technique can be attributed to the low contrast between the tumor and the surrounding tissue. This is especially evident in the radiographically dense breast, which contains large amounts of glandular and fibrous tissue, with similar X-ray attenuation as the lesion.
For the patient safety and comfort there are few additional drawbacks with X-ray mammography. Among patients it is sometimes seen as an uncomfortable and painful examination due to the need for breast compression. Exposing patients to ionizing radiation on a regular basis within mammography screening programs is also not entirely satisfactory. Even if the radiation dose is kept very low there is a risk that the repeated exposure to X-rays will induce carcinoma in the breast.
There exist complimentary imaging methods that avoid the ionizing radiation and the uncomfortable breast compression. The most important are ultra sound imaging and contrast-enhanced magnetic resonance imaging (MRI). None of these methods are suitable for or have been used in a mass screening program although they are in some cases useful later in the process of diagnosing malignant breast cancer, and for evaluating dense breasts. Other methods such as X-ray computed tomography, digital subtraction angiography, diaphanography and imaging using radio nuclides only has a small role in breast cancer examination today.